vfs: fix freevnodes count update race against preemption

vdbatch_process leaves the critical section too early, openign a time
window where another thread can get scheduled and modify vd->freevnodes.
Once it the preempted thread gets back it overrides the value with 0.

Just move critical_exit to the end of the function.

sys/kern/vfs_subr.c

@@ -3324,10 +3324,10 @@ vdbatch_process(struct vdbatch *vd)
 		vp->v_dbatchcpu = NOCPU;
 	}
 	mtx_unlock(&vnode_list_mtx);
-	critical_exit();
 	vd->freevnodes = 0;
 	bzero(vd->tab, sizeof(vd->tab));
 	vd->index = 0;
+	critical_exit();
 }
 
 static void